The invention relates to methods for producing cement clinker, having the following steps: preheating calcium carbonate-containing raw meal in a preheating stage which is heated by exhaust gases from a sintering stage which follows in the gas flow direction, deacidifying the preheated raw meal, sintering the deacidified raw meal into cement clinker in a sintering stage, cooling the cement clinker from the sintering stage in a cooling stage which cools the cement clinker by means of a gas.
In the methods for producing cement which are carried out most often throughout the world, a calcium carbonate-containing initial material in the form of limestone is freed of CO2 formally by the supply of heat and is thereby converted into unslaked lime, calcium oxide, and is subsequently sintered by the supply of even more heat, in the presence of silicate-containing rock, into cement clinker which is composed of various calcium silicate phases and constitutes the principal fraction of customary cement. In this case, heat energy of between 2850 and 3350 kJ is used per kg of cement clinker. The heat quantity required for this purpose is usually generated from the combustion of carbon-containing fuel. Combustion, on the one hand, and the formal freeing of CO2 from the limestone, on the other hand, together form an intensive CO2 source, the released CO2 hitherto being introduced into the free earth's atmosphere. The CO2 emission thereby generated makes an appreciable contribution to the overall anthropogenic CO2 emission on earth. It is known since then that CO2 is the main cause of an anticipated greenhouse effect which leads to the undesirable warming of the earth's atmosphere. The endeavor, therefore, is to reduce the CO2 emission substantially.
In order to reduce the introduction of CO2 into the earth's atmosphere due to the production of cement, it is necessary to rely on preventing the released CO2 from escaping into the earth's atmosphere by storing it in underground caverns. Such caverns are, for example, natural gas or petroleum deposits which have for the most part been emptied. Since, in the conventional method for producing cement, very large quantities of CO2 occur, which are mixed with even much larger quantities of nitrogen from atmospheric air, storage, along with compressing the exhaust gas and transferring it to the deposit, is scarcely possible in economic terms.
In the hitherto known method for producing cement, it is customary to fine-grind the calcium carbonate-containing initial material into what is known as raw meal and then first to heat it in a preheater. In the preheater, the raw meal falls in countercurrent to the gas flow direction through the hot exhaust gases of a cylindrical rotary kiln, in order first to heat by the waste heat the large quantities of limestone to be burnt. Depending on the configuration of the plant, there is then provision for deacidifying the raw meal in a cylindrical rotary kiln and sintering it into limestone in one step or for carrying out deacidification and sintering in separate plant parts. The gases which heat the raw meal and are composed of nitrogen, CO2, small quantities of CO, nitrous gases and further combustion gases are then, in many plants, conducted through a heat exchanger to separate the heat still remaining in the exhaust gases and are then released into the free earth's atmosphere.
Since the exhaust gas quantities occurring in order to prevent CO2 emission are very large, European patent application EP 1 923 367 A1 proposes to modify the hitherto known method for producing cement. According to the proposal of the last-mentioned patent application, preheating and deacidification are to be carried out in spatially separate regions of the plant, the exhaust gases from deacidification being circulated, along with a high degree of enrichment of CO2, so that deacidification is carried out in a CO2 atmosphere. The chemical balance lies in this case on the side of unslaked lime due to the heat introduced. By contrast, as is known, the exhaust gases from a cylindrical rotary kiln are used to preheat the raw meal and are then discarded by being released. In order to utilize the residual heat from the cylindrical rotary kiln exhaust gases after heat exchange with the raw meal, the last-mentioned patent application proposes to cool down the exhaust gas with the aid of a heat exchanger in favor of heating water for energy generation, during which steam occurs in the second circuit of the heat exchanger and is to be used for driving steam turbines.
The method referred to in the last-mentioned patent application therefore still causes the CO2 occurring during the combustion of carbon-containing fuels to escape into the atmosphere, approximately 40% of the entire fuel burnt in the plant usually being converted in the cylindrical rotary kiln. It would be ideal if the CO2 escaping here could also be captured and stored.